1. Field of the Invention
The present invention relates to the field of power generation and industrial boiler design, including Kraft process recovery boilers or soda process recovery boilers used in the pulp and paper industry. In particular, the present invention provides a new and useful dual pressure recovery boiler installation which employs an enhanced steam cycle with reheat to achieve a large increase in electrical generation with various turbine cycles while overcoming traditional lower furnace material limitations.
2. Description of the Related Art
The recovery boiler is utilized by the pulp and paper industry to provide a means for recovery of certain chemicals created as part of the pulping and papermaking process, electrical generation through a steam turbine, and production of process steam used by the mill in the pulping and papermaking process.
For a general discussion of chemical and heat recovery in the pulp and paper industry, and the particular aspects of the alkaline pulping and chemical recovery process, reference is made to Steam/its generation and use, 41st Ed., Kitto and Stultz, Eds., Copyright© 2005, The Babcock & Wilcox Company, Chapter 28.
Referring to the drawings of the present application, FIG. 1 is sectional side view of a known Kraft recovery boiler manufactured by The Babcock & Wilcox Company. The two main functions of a Kraft process recovery boiler, soda process recovery boiler, or simply, “recovery boiler”, are to burn the organic portion of black liquor (a by-product of chemical pulping) to release energy for generating steam and to reduce the oxidized inorganic portion of black liquor in a pile, or bed, supported by the furnace floor. The molten inorganic chemicals in the bed, known as smelt, are discharged to a tank of water where they are dissolved and recovered as green liquor.
The recovery boiler illustrated in FIG. 1 comprises a furnace 10 which is typically rectangular in cross-section, having enclosure walls 12 formed of water or steam-cooled tubes. The black liquor is fed into a lower portion of the furnace 10 through one or more black liquor spray nozzles 14 which spray the black liquor into the furnace 10 through openings in the enclosure walls 12. The furnace 10 is generally rectangular in cross-section, and has a front wall 16, a rear wall 18 and two side walls 20. Combustion air is introduced into the recovery boiler furnace 10 via air ports at staged elevations above a floor 22 of the furnace 10. These elevations are—primary air 24, secondary air 26, and tertiary air 28, as shown in FIG. 1. The gases generated by combustion rise out of the furnace 10 and flow across convection heat transfer surfaces. Superheater (SH) surface 30 is arranged at the entrance to the convection pass, followed by steam generating (Boiler Bank) surface 32 and finally economizer (EC) surface 34. A furnace arch or nose 37 uniformly distributes the gas flow entering the superheater surface 30.
The potential for corrosion in the lower furnace of recovery boilers is a significant issue. As is known to those skilled in the art, recovery boilers operate with the lower furnace in a reducing atmosphere (reduced oxygen) environment. The tubes forming the lower furnace water-cooled enclosure walls 12 which are exposed to this reducing atmosphere experience extremely accelerated corrosion rates. As a result, the lower furnace enclosure walls 12 must have additional protection from corrosion.
Early designs to enhance corrosion resistance employed cylindrical pin studs welded to the tubes in the reducing zone of the lower furnace. The pin studs held solidified smelt, forming a barrier to the corrosive furnace environment. The traditional pin stud arrangement later evolved into the use of composite or bimetallic tubes, as the design pressure of recovery boilers increased to above 900 psig. The composite tubes are comprised of an outer protective layer of AISI 304L stainless steel and an inner core layer of standard American Society for Testing and Materials (ASTM) A 210 Grade A1 carbon steel. The composite tube inner and outer components are metallurgically bonded. The outer layer of austenitic stainless steel, which is also used to cover the furnace side of the carbon steel membrane bar, protects the core carbon steel material from furnace corrosion. Other methods used for lower furnace corrosion protection include: chromized carbon steel tubes, chromized pin studs, carbon steel pin studs, metallic spray coatings, high density pin studs, 304L, Alloy 825 and Alloy 625 composite tubes, and weld overlay of carbon steel tubes. All these approaches are extremely expensive.
A recent solution to the problem of lower furnace corrosion is disclosed in U.S. Pat. No. 7,243,619 to Graves et al., which provides a dual pressure boiler system having a furnace that is divided into two sections—a bottom low pressure furnace and a top high pressure furnace. The bottom furnace operates as a separate low pressure natural circulation steam generating system. The top furnace operates as a high pressure natural circulation steam generating system. Since the water tubes in the bottom furnace operate at lower temperatures and lower pressures, they are less susceptible to corrosion.
FIG. 1A of the present disclosure is a schematic diagram of this dual pressure recovery boiler, generally designated 100. The low pressure bottom section 110 and the high pressure top section 110′ form separate natural circulation systems. Each section 110, 110′ has its own dedicated steam drum 112, 112′ for separating saturated steam from water, pump 114, 114′ for pumping feed water to the steam drum 112, 112′, and superheater 120, 120′ for increasing the temperature of the saturated steam which exits the steam drum 112, 112′. Tubing 118 routes the saturated steam to the low pressure superheater 120, and then to a plant steam header 122. The separated water from the low pressure steam drum 112 flows in piping 128 into the low pressure bottom section 110 of the boiler 100. The water enters into and circulates in furnace wall tubes forming section 110 and then re-enters the low pressure steam drum 112 as a steam-water mixture. The natural circulation system in the high pressure top section 110′ operates similarly but at higher temperatures and pressures. The pump 114′ feeds water to heat exchanger or economizer 117 which is fluidically connected downstream from the pump 114′ before the high pressure steam drum 112′. The economizer 117, in turn, discharges the water to the high pressure steam drum 112′. Steam is separated from the circulating water and routed via tubing 118′ to the high pressure superheater 120′. From the high pressure superheater 120′, the steam flows to turbine/generator 124 to produce electricity. Water from the high pressure steam drum 112′ flows in piping 128′ into the high pressure top section 110′, circulates through the upper furnace walls of top section 110′ and the water-steam mixture is conveyed to the high pressure steam drum 112′.
Pulp and paper mills are constantly seeking ways to increase the power output and efficiency of steam generators. Raukola et al., in a technical paper titled “Increasing Power Generation with Black Liquor Recovery Boiler” presented at the 2002 TAPPI Fall Conference & Trade Fair, describe several approaches. These include: increased dry solids content of the black liquor to increase boiler efficiency; air preheating with extraction steam from the steam turbine; taking sootblowing steam from extraction steam from the steam turbine, rather than from after the primary superheater, in order to extract more useful work from the steam; in back-pressure steam turbine installations, not throttling the back-pressure steam in order to increase feedwater temperature; employing high-pressure feed water preheaters using extraction steam from the steam turbine; increasing main steam temperature and pressure (noting, however, that corrosion of the furnace walls and in the superheater area are the biggest concerns related to this approach); providing a reheater arrangement where the main steam, after expanding through the turbine, is sent back to the boiler to be superheated again before the next turbine stage; employing a condensing steam turbine instead of a back-pressure steam turbine; and employing heat recovery after the electrostatic precipitator to replace back-pressure steam used normally for preheating and thus releases steam to be used for power generation with the condensing turbine.
U.S. Patent Application Publication US 2006/0236696 A1 to Saviharju et al. discloses a spent liquor recovery boiler which is provided with a reheater for reheating steam from the high-pressure part of the turbine. The recovery boiler has a conventional furnace with the exception of the provision of at least one cavity preferably located at an upper portion of the recovery boiler furnace front wall. The reheater has a first part and a second part, the first part being located in the stream of flue gas between the superheater and a boiler bank, with the second part of the reheater being located within the cavity. The cavity may also include a superheater section. Flue gases formed in the cavity enter the furnace after passing across the second reheater part and superheater section.
U.S. Pat. No. 5,603,803 to Raak discloses a method and apparatus for recovering heat in a soda liquor recovery boiler. The boiler walls are formed of water-cooled tubes connected to the water/steam circulation system of the boiler. The lower section of the boiler is defined by water tubes connected to a separate water circulation system of a forced, rather than natural circulation, type, and has a lower pressure than that of the actual boiler. The cooling circulation in the lower section of the furnace is arranged by using a separate water circulation system. The heat recovered to a separate water circulation system may be used, e.g., for heating the boiler feed water, e.g., in a separate heat exchanger, which is connected with the water circulation system by a separate cooling circulation system, whereby the heat released from the cooling of the lower section of the furnace is recovered. Thereby, it is possible to maintain the temperature of the medium flowing in the lower section of the separately cooled furnace nearly constant by regulating the cooling effect of the heat exchanger in the cooling circulation system. The temperature of the cooling medium flowing in the cooling circulation system of the boiler according to the invention is preferably regulated so that it causes the thermal expansion of the separately cooled lower section of the furnace to correlate with the thermal expansion of the walls within the boiler water/steam circulation systems, i.e., no sealing problems exist between the separately cooled lower section and the other furnace structure and no gas or chemical leakages occur between the parts.
FIG. 2 is a schematic illustration of a known pulp mill recovery boiler and steam turbine installation employing a conventional superheat (SH) cycle, and generally referred to as 200. Temperatures (degrees F.), pressures (pounds per square inch gage or absolute—psig, psia) and flow rates (thousands of pounds per hour—kpph) are provided merely for illustrative purposes. As illustrated therein, recovery boiler 202 comprises a furnace 210 having enclosure walls 212 formed of fluid-cooled tubes which generally contain a water-steam mixture. The black liquor is fed into a lower portion of the furnace 210 and combusted with air. The gases generated by combustion rise out of the furnace 210 and flow across convection heat transfer surfaces, and which include superheater (SH) surface 230 and economizer (EC) surface 234. The water-cooled furnace enclosure walls 212 cool the combustion gases and generate a steam-water mixture therein. A furnace arch or nose 237 uniformly distributes the gas flow entering the superheater surface 230.
Feedwater pump 236 provides feedwater to the economizer 234 via line 238. Flue gases from combustion of the black liquor pass across the economizer 234, preheating the incoming feedwater which is conveyed via line 240 to steam drum 242. The hot combustion flue gases transfer heat to the enclosure walls 212, generating a water-steam mixture therein which is also conveyed upwardly therethrough to the steam drum 242 via risers 244. Separation devices (not shown) within the steam drum 242 separate the water from the water-steam mixture. The feedwater mixes in the steam drum 242 with the separated water and then this mixture is conveyed to the lower portion of the furnace 210 via downcomers 246. Saturated connections 248 convey the steam from the steam drum 242 to the superheater 230, where the steam is superheated. The superheated steam is then conveyed via line 250 to steam turbine 252 which is advantageously connected to an electric generator (not shown) for producing electricity. The superheated steam expands through the turbine 252, causing the turbine rotors to spin, thereby causing the electric generator connected thereto (not shown) to generate electricity. A portion of the steam is conveyed via line 254 to a condenser 256. The majority of the steam exiting from the turbine 252 is extraction steam used to supply various plant process requirements. For example, line 258 conveys 150 psia process steam to header 260 and this steam is then conveyed via one or more lines 262 to various plant processes. Similarly, line 264 conveys 75 psia process steam to header 266 and this steam is then conveyed to one or more lines 268 to other various plant processes. Condensate from condenser 256 is then conveyed via line 270 to deaerator 272 which, in turn, provides the condensate via line 274 to feedwater pump 236, completing the fluid cycle.
Recovery boilers with reheat steam turbine cycles are known, as disclosed by Saviharju et al. However, that design still deals with the lower furnace corrosion concerns of the prior art in conventional fashion; i.e., the steam pressure in the furnace walls low enough such that excessive corrosion does not take place in the water-cooled tube walls of the furnace. The saturation temperature in the water-steam emulsion plus the temperature difference due to incoming heat flux from the tube surface into water is less than 400-500 C. (752 F.-932 F.), typically less than 400 C. (752 F.), which is the tube surface temperature. Raak discloses a soda liquor recovery boiler where a lower section of the boiler has a lower pressure than that of the actual boiler and is defined by water tubes connected to a separate water circulation system of a forced, rather than natural circulation, type. Heat may be recovered for preheating the boiler feed water or combustion air, in the separate water circulation system; however, the separate water circulation system is not in communication with the water/steam circulation system, and the purpose of the construction is not to provide increased electrical generation capacity but rather to maintain the temperature of the medium flowing in the lower section of the separately cooled furnace nearly constant by regulating the cooling effect of the heat exchanger in the cooling circulation system to address thermal expansion concerns that might cause sealing problems between the separately cooled lower section of the furnace to correlate with the thermal expansion of the walls within the boiler water/steam circulation systems. Graves et al. provides a solution to the problem of lower furnace corrosion by providing a dual pressure boiler system having a furnace that is divided into two sections—a bottom low pressure furnace and a top high pressure furnace. However, Graves et al. does not teach or suggest any way to enhance the electrical generating capability of the overall plant. The superheated steam from the high pressure superheater is routed to a turbine generator for producing electricity. The superheated steam from the low pressure superheater is piped to a plant steam header for use as process steam.
It is apparent that an improved recovery boiler design which provides increased operating efficiency and electrical generation output while reducing the potential for lower furnace corrosion would be welcomed by the industry.